Various electronic instruments, including, for example, protective relays for power systems and electricity meters, sample analog input signals by means of an analog-to-digital converter to produce digital signals which are then processed to produce specific information from the input signals which relates to the function of the instrument. Appropriate action is taken, relative to the instrument, as needed, in response to that information. For instance, in a protective relay, if the information from the input signals indicates a fault on the power line, the instrument may trip the current breaker for that portion of the line.
In certain situations/system arrangements, it is important that sampling of the analog input signals be made simultaneously by multiple instruments, again for instance, simultaneous sampling of line voltage and current input signals in a plurality of protective relays. This presents a challenge when the instruments are not all located in close physical proximity. A number of solutions concerning the synchronous sampling of inputs to multiple electronic instruments, however, are known. One such system uses a repeating time pulse, derived from the Global Positioning System.
Many of the electronic instruments which utilize sampling functions also have a time-of-day clock and calendar which the instrument uses to time-tag particular reports or other information which the instrument generates. One example of such a report is an oscillograph report from a power system protective relay; another example is a revenue report from an electricity meter. It is important that the time-of-day clocks in the multiple devices are synchronized to all report the same time, at the same instant, on time. Many current devices use an encoded time-of-day signal, such as an IRIG-B signal, to synchronize their time-of-day internal clocks.
As presently configured, most such instruments use two control signals to accomplish desired synchronization, one control source being a periodic pulse by which each device synchronizes its sampling of input signals to be measured, and the other being a nonperiodic encoded time-of-day signal to which the device synchronizes its time-of-day clock.
While the use of two separate control signals has proved to be workable, it is desirable to have a single control signal to synchronize both data sampling (or other periodic action) and the time-of-day clock in electronic instruments.